This invention relates generally to fiber optic hydrophones and particularly to a solid interferometric fiber optic hydrophone. Still more particularly, this invention relates to a solid interferometric fiber optic hydrophone that is suitable for operation at high pressures with high, linear acoustic sensitivity and low sensitivity to linear acceleration.
Heretofore, fiber optic hydrophones have comprised two concentric hollow mandrels, each wrapped with a length of optical fiber that forms one leg of an optical interferometer. The outer mandrel typically is thin-walled so that its radius changes in response to incident acoustic pressure. A sealed air cavity is formed between the two mandrels. The inner mandrel is typically either thin-walled, with its interior exposed to the ambient pressure so that its radius would change in the opposite sense from that of the outer mandrel under acoustic pressure "push-pull" configuration), or relatively thick-walled, and possibly filled with a solid potting compound to serve as a "reference arm" for the interferometer.
The thin sensing shells backed by an air cavity are very compliant and provide a sensitive hydrophone. The cylindrical shape provides for a low net change in fiber length (i.e. low sensitivity) due to linear accelerations, which is desirable in a hydrophone. The ability to survive submersion to great depth must be provided by ensuring that the outer shell is thick enough to resist being crushed by the water pressure.
This depth survival requirement limits the compliance that the mandrel can exhibit in response to acoustic signals. Hence, with the prior art hydrophone technology it is difficult to deploy a sensitive hydrophone in an array that must survive at depth.
Production of these hydrophones is also time consuming and costly because the air cavity between the two mandrels must be sealed. This seal must allow for passage of fiber through it to the outside of the inner mandrel. This is a delicate assembly process, and leaking and fiber breakage at this seal is not an uncommon failure mechanism of these instruments.
The air cavity can also support acoustic resonances. The relatively low speed of sound in air and the high damping of acoustic waves in the air cavity can cooperate to establish these resonances within the acoustic detection bandwidth. This limits the frequency range of linear operation of the hydrophone, and can be problematic in some hydrophone designs.